This invention relates to improvements in the detection of certain nitrogen-containing gases using chemiluminescence.
Chemiluminescence is the emission of light that results from a chemical reaction. A chemical reaction between substances generally is accompanied by the release of energy. Frequently this energy is manifested in the form of heat. Occasionally, however, it is accompanied by the production of light in the visible or infra-red portion of the spectrum.
The detection of chemilum inescence when two substances are brought together can serve as a method for detection and analysis of one of the substances. For example, the occurrence of chemiluminescence when substance "B" is added to substance "A" may provide evidence of the presence of substance "A", while the intensity of the chemiluminescence may provide information on the amount of "A" present. There is, for example, a commercially available instrument that operates on this principle and which determines the concentration of nitric oxide (NO) from the chemiluminescence produced when it reacts with ozone.
A difficulty frequently encountered with chemiluminescence as a method for chemical analysis is its lack of specificity. Substances other than substance "A" also may produce chemiluminescence in the presence of substance "B". Although the wavelengths of the emitted light are characteristic of the particular chemical reaction, the range of wavelengths involved may overlap wavelengths emitted from reactions involving chemiluminescence between substance "B" and other interfering substances. In such cases the interfering substances first must be removed before the analysis is performed.
Luminol (5-amino-2,3-dihydro-1,4-phthalazine dione) is known to chemiluminesce with a number of oxidizing agents in alkaline solution when metal ion catalysts are provided (White, E. H. in "Light and Life" 1st Edition McElroy, W. D., Glass, B. Eds.; Johns Hopkins Press: Baltimore, 1962, p. 183). This technique has been used to measure hydrogen peroxide (H.sub.2 O.sub.2) as well as other oxidizers produced by biological systems (Selz, W. R. Methods Enzym. 1978, 57, 445, Schroeder, H. R. et al Methods Enzym. 1978, 57, 425) and to measure metal ion concentrations when excess H.sub.2 O.sub.2 is added (Seitz, W. R.; Hercules, D. M. in "Chemiluminescence and Bioluminescence"; Cormier, M. J., Hercules, D. M., Lee, J. Eds; Plenum Press, New York p. 427). Kok et al (Environ. Sci. Tech. 1978, 12, 1072; 1978, 12, 1077) devised a system for measuring H.sub.2 O.sub.2 based on the H.sub.2 O.sub.2 /luminol/metal ion reaction. All of these reactions involving chemiluminescence occur in the liquid phase and depend upon the presence of metal ion catalysts in the solution.
Anderson et al (U.S. Pat. No. 3,659,100, August 1970 and U.S. Pat. No. 3,700,896, October 1972) described a system based on luminol/H.sub.2 O.sub.2 chemiluminescence with the catalyst being a gas in equilibrium with the luminol solution instead of a metal ion. Anderson et al further disclosed that the system can be used as a detector of NO.sub.2, O.sub.3 and SO.sub.2 by using various inlet traps to distinguish between these gases.
Maeda et al (Anal. Chem. 1980, 52, 307) described the development of a detector for nitrogen dioxide (NO.sub.2) based on the reaction involving chemiluminescence between NO.sub.2 and luminol. Metal ions were carefully removed from the solution eliminating interferences encountered with other oxidants which required the presence of metal ion catalysts.
The chemiluminescence between NO.sub.2 and luminol is a process which occurs on the surface of the liquid. This is in contrast with chemiluminescence involving other oxidants, such as H.sub.2 O.sub.2, which occur from reaction in the bulk phase of the liquid. Maeda et al described in the aforementioned publication a reactor in which the gas was drawn over the surface of a pool of luminol solution. A photomultiplier was located close to the surface of the pool and monitored the intensity of the chemiluminescence. This Maeda reactor design has two serious drawbacks. It is very sensitive to both, movement during sampling and positioning of the cell, to keep it level. Any wetting of the wall of the reactor above the pool leads to changing surface volume and changing signal. In addition, this design exhibited a slow response to changes in concentration of NO.sub.2.